1. Field of the Invention
This invention relates to thermal conductivity measurement devices, and in particular, to precision measurement devices for measuring the thermal conductivity of a fluid, such as a gas, to detect compounds within the fluid.
2. Description of Related Art
Gas chromatographs are used to determine the chemical composition of a sample, which may be gaseous or a vaporized liquid. The term gas will hereinafter be used to include a vapor. In one type of gas chromatograph, a sample is sent through a separation column. A typical separation column is a long capillary tube with a coated interior. Different chemical compounds in the sample travel through the separation column at different rates and leave the separation column at different times. As compounds leave the separation column, they are carried by a carrier gas past a detector. One commonly used carrier is helium, but other gases may be used. The detector detects compounds in the carrier gas by measuring changes in the properties of the effluent gas. When a change in the gas property occurs, the timing of the change indicates the type of the compound passing the detector, and the magnitude of the change indicates the quantity of the compound.
One type of detector used with gas chromatographs is a thermal conductivity detector, which detects changes in the thermal conductivity of the effluent gas. When a compound is mixed with the carrier gas, the thermal conductivity of the mixture is usually different from that of the pure carrier gas. A thermal conductivity detector provides a measure of the change in the thermal conductivity of the carrier gas and thereby provides a measure of the presence and amount of various compounds.
FIG. 1 shows a typical prior art sensor circuit 10 used in a thermal conductivity detector of a gas chromatograph. The sensor circuit 10 includes a metal filament 12, such as a platinum wire, placed in a cavity 14. The effluent from a gas separation column along with a carrier gas fills the cavity 14 and flows along a path 16 past the filament 12. The filament 12 has a resistance R.sub.S which depends on its temperature and is heated using an electric current I.sub.1. In the case of the filament 12 being a platinum wire, the resistance of the filament 12 is proportional to its temperature.
Heat generated by the filament 12 is removed partially by the flow of the effluent but primarily by thermal conduction through the gas to the walls 18 of the cavity 14, thus lowering the resistance of the filament 12. By effectively measuring the change in resistance of the heated filament 12, the change in thermal conductivity of the flowing gas may be determined.
In application, several problems arise that can cause the output of a detector to change even if the composition of the gas remains constant. One problem is caused by changes in the temperature of the walls 18 of the cavity 14. Another problem is caused by changes in the temperature of the carrier gas. Another problem is electronic drift of the energizing voltage which controls the current through the filament 12. With the sensitivity required of a detector, even thermoelectric potentials generated in electrical connections may affect the detector. Changes in the voltage offset of the amplifiers used to measure changes in the resistance of filament 12 are still another problem.
One technique for trying to avoid some of these problems is using the filament 12 in a bridge circuit employing a control filament 22 (FIG. 1) which is ideally identical to the filament 12 and is located in a cavity 24 similar to the cavity 14 but containing only a pure carrier gas. A variable resistor R.sub.b is used to match the resistance of a fixed resistor R.sub.a. A differential amplifier 26 detects an unbalance in the bridge. A DC voltage supply is used to heat up the filaments 12 and 22 to a temperature typically between 5.degree. to 100.degree. C. above the temperature of the cavity walls 18.
If the thermal conductivity of the effluent in the cavity 14 is different from that of the pure carrier gas in cavity 24, the bridge becomes unbalanced, and a change in the amplifier's output voltage V.sub.A indicates the detection of a change in thermal conductivity of the gas in the cavity 14. Common variations in the wall temperature of the cavities 14 and 24 as well as common variations in the temperature of the carrier gas change the resistances of the filaments 12 and 22 equally and do not affect V.sub.A. Similarly, a fluctuation in V.sub.DC does not affect V.sub.A if the cavities 14 and 24 and filaments 12 and 22 are identical.
A problem with this method is that two different sensors cannot be made exactly the same and would, therefore, not react identically to identical changes in their ambient environment. And, even if the sensors could be made to be initially identical, the properties of the sensors change with time, producing a bridge imbalance with common changes in the ambient environment of the filaments 12 and 22.
Accordingly, improved methods and structures for detecting thermal conductivity are needed to avoid the detrimental effects due to imprecise matching of the sensor resistor in a bridge type sensor circuit.